Worldwide energy demands together with high value of cereal grains provide strong incentives to produce ethanol from other biomass feedstocks. Ethanol has a number of uses, but is used primarily as a fuel or a fuel additive to gasoline to boost octane, reduce pollution, and/or to partially replace gasoline and reduce crude oil requirements.
Ethanol may be produced from a variety of biomass feedstocks. As used herein, biomass refers to any organic material that is renewable. For example, ethanol may be produced from a plant-based feedstock derived from an energy crop. Exemplary energy crops include corn, sugarcane, millet and so on. The fruits and/or seeds of an energy crop typically comprise a large portion of starch, which is readily fermentable into ethanol by conventional processes.
Conventional processes known in the art for producing ethanol from energy crops, such as corn, include dry mill or wet mill fermentation processes. See for example, CORN, Chemistry and Technology, Stanley A. Watson and Paul E. Ramstad, editors, 1987, Published by the American Association of Cereal Chemists, Inc., St. Paul, Minn., USA, which is incorporated herein by reference for all relevant purposes. For example, dry mill ethanol production uses the starch portion of corn kernels, which is about 70% of the kernel. The starch component is converted by hydrolysis to sugars, which are then fermented to form ethanol. The ethanol is then recovered from the fermentation mass by means known in the art.
Ethanol may also be prepared from a variety of lignocellulosic biomass materials including crop residues such as corn stover, rice straw, wheat straw, and bagasse; logging and mill residues such as wood chips, saw dust, and pulping liquor; grasses such as switchgrass; trees such as poplar and willow; and plant-derived wastes such as household garbage and paper products. Thus, lignocellulosic biomass is a readily available and relatively inexpensive substrate for the preparation of sugars, which may be fermented to produce alcohols such as ethanol.
Generally, the preparation of ethanol from lignocellulosic biomass involves (1) liberating cellulose and hemicellulose from lignin and/or increasing the accessibility of cellulose and hemicellulose to enzymatic or chemical hydrolysis, (2) depolymerizing carbohydrate sugars of hemicellulose and cellulose to free sugars, (3) fermenting the sugars to ethanol, and (4) separating the ethanol from the fermentation mass.
Regardless of the type of biomass feedstock, ethanol is typically separated from the fermentation mass by distillation. The bottoms of the distillation operation, which is referred to herein as stillage, consists primarily of protein, lignin, inorganics (e.g., SiO2, CaO, MgO, KO, Fe2O3, P2O5, Al2O3, etc.), unreacted or partially reacted carbohydrates (e.g., cellulose or hemicellulose), and residual sugars. These values have a variety of uses. For example, when ethanol is produced from fermentation of cereal grains, such as corn, the process yields protein-containing co-products that are known in the art as Wet Distillers Grains (WDG), Dried Distillers Grains (DDG), Wet Distillers Grains Plus Solubles (WDGS), or Dried Distillers Grains plus Solubles (DDGS), which are collectively referred to herein as distiller's grain. Distiller's grains are typically used for animal feed. Furthermore, lignin, which is a complex random polyphenolic polymer, can be used as a fuel or fuel additive and can serve as a raw material for a variety of products including wood adhesives, flame retardants, slow-release agents for agricultural and pharmaceutical products, surfactants, asphalt/concrete extenders, drilling mud, and plastics. Inorganics obtained from a fermentation mass may be used in fertilizer formulations or as adsorbents or filtration media. Carbohydrates and residual sugars can be further utilized as sources of sugar in a fermentation process for the production of ethanol.
Various processes for recovering lignin and/or silica from biomass have been disclosed. For example, Lucas et al., U.S. Pat. No. 5,735,916, disclose a process for producing lignin fuel, silica/caustic oxide, cellulose, and cellulose derivatives from plant biomass. The process includes (1) milling or grinding plant biomass; (2) extracting sugar, soluble salts, soluble plant proteins, and soluble polypeptides from plant biomass at a pH between 3-5 using a mild acid solvent and sending the extract to a fermentation process; (3) filtering off the remaining solid material; (4) dissolving lignin and silica from the solid material with a caustic hydroxide solution; and (5) separating lignin from the caustic silicate solution by ultrafiltration.
Farone, U.S. Pat. No. 5,597,714, describes a process for producing sugars for various uses by concentrated acid hydrolysis of biomass and a process for removing silica or silicates from solids remaining following the acid hydrolysis of biomass. In this process, biomass is first decrystallized and then hydrolyzed with acid to convert cellulose and hemicellulose to sugars. Following hydrolysis, the sugar solution is separated, by means of a belt press, from any remaining solid materials, which include lignin. The remaining solid materials are treated with a metal hydroxide solution at a pH above 12 to extract a solution containing silicic acid. Thereafter, silica is precipitated from solution at a pH around 10 and filtered from the solution. Throughout the process lignin remains insoluble in the aqueous medium of the process. Insoluble lignin together with other residual solid materials are recovered by belt press for use as a wet lignin fuel.
While these processes may improve on the economics of a process for the preparation of ethanol from biomass by recovering or removing values from a pretreated biomass feedstock, there remains opportunity for further improvement by recovering values from a fermentation mass, such as stillage, obtained in producing ethanol. Moreover, the need exists for a process that recovers and fully utilizes carbohydrates, lignin, inorganics, and residual sugars obtained from a fermentation mass that can be practiced on a commercial scale to further improve economics of a process for the preparation of ethanol from biomass feedstocks.